A typical WDM-PON system has passive optical components, e.g., fibers, splitters and couplers, to guide traffic between an optical line termination (“OLT”) unit located at a Central Office (“CO”) and remote Optical Network Units (“ONUs”) located at customer premises. The WDM-PON system may use a broadband light source (“BLS”) that produces light having a continuous spectrum and uniform power spectral density.
FIG. 1A shows a block diagram of a typical broadband light source. As shown in FIG. 1A, broadband light source 100 consists of a gain element 101, a gain flattening filter (“GFF”) 102, and an isolator 103. Gain element 101, such as an erbium doped fiber amplifier (“EDFA”) or semiconductor optical amplifier (“SOA”) is used to generate amplified spontaneous emission (“ASE”) that is continuous over a specified wavelength band. GFF 102 is typically used to flatten the continuous ASE spectrum. Isolator 103 is used to prevent reflections from affecting the output power and spectral shape of the light from BLS 100.
FIG. 1B shows a diagram illustrating dependence of an output 112 of BLS 100 on a wavelength λ. As shown in FIG. 1B, output 112 of BLS 100 has a continuous spectrum. The output 112 of BLS 100 is uniform over a continuous range λ2-λ1 of wavelengths λ, as shown in FIG. 1B.
Broadband light source 100 with a continuous spectrum output typically causes many problems that limit the performance of the WDM-PON system. One of the problems is the increased optical crosstalk between adjacent optical channels in a WDM-PON. Another problem is that the peak spectral density in each wavelength channel is limited by the total output power that can be extracted from the gain element in a BLS device. Further, the wavelength accuracy of the channel filtering in the WDM-PON typically can be difficult to maintain in low-cost volume manufacturing. Additionally, the relative intensity noise (“RIN”) on the BLS signal can limit the system performance.